There's no straightforward answer as different ISPs will have different anti-spam measures.
However, if your 'shared' email server a relatively busy one, then yes, there is the proportionally increased risk of people sending out a larger number of spam and virus messages.
If your ISP then does not deal with those spammers in a timely and efficient manner, then they increase the risk of getting themselves on blacklists.
I wouldn't say go as a far as a dedicated server (just yet), but perhaps try a different shared service such as smtp.com (never used myself, but know some others who have).
Last edited by mixture; 21st Jun 2013 at 07:59.